One technique used in direction of arrival systems operating in head wearable devices is to combine microphone output signals from the left and right sides of the head to determine the delay between sounds present in the microphone outputs. When sounds emanate from the medial (front or rear) region of the wearer, there is little delay between the microphone output signals. However this delay is largest when sounds emanate from the one side of the head. The delay increases monotonically from the medial region to either lateral region. This monotonic increase can be translated into direction of arrival of sounds with reference to the midline location between both ears.
Another technique relies on the shadowing effect of the human head. The head casts a shadowing effect for sounds located on opposite sides of the head. Due to this head shadowing effect there can be more than 20 dB level differences between microphone output signals. The level difference also decreases monotonically as the sound moves from the side to the midline location between both ears. These two basic mechanisms have been used in direction of arrival algorithm based on wearable hearing devices.
Numerous techniques have been tried to compare left and right microphone output signals and derive a direction of arrival estimate. These techniques include; Correlation, Maximum Likelihood (covariance minimisation), Multiple Signal Classification (MUSIC), Estimation of Signal Parameters using Rotational Invariance Techniques (ESPRIT) or Eigen decomposition, and Matrix pencil using an array manifold or triangulation. However, these techniques only operate successfully in relatively quiet environments.
For instance, a common technique for direction of arrival relies on sensory microphone arrays whereby the cross-correlation between the microphone output signals is calculated to determine the delay at which the maximum output power or peak occurs. In the presence of multiple sound sources these systems fail to continuously and accurately estimate the direction of arrival of a target sound present in the environment. Instead the estimates reflect the direction of arrival of dominant sounds. However due to temporal fluctuation characteristics of different sound sources the dominant sound typically changes from time to time, creating ambiguities in the estimates.
This is a particular problem for applications in which the constant and accurate detection of a target sound sources present in an arbitrary spatial location in space is required. For example, head-wearable devices such as hearing aids and hearing protectors may integrate bilateral beamformer technology to improve the Signal-to-Noise (S/N) ratio available to listeners and in the process remove the localisation cues. In such systems the direction of arrival of a desired target sound may be needed to reconstruct the localisation cues for listeners using, for instance, virtual auditory space reconstruction techniques.
There remains a need for improved direction of arrival techniques.